An optical coherent receiver includes a local oscillator (LO) laser for generating an LO signal at an LO frequency. The LO signal is added to a received input optical signal at a carrier frequency, and the combined lightwave is detected by a photodetector. The photodetector outputs a detected signal at an intermediate frequency (IF), which may be the sum or the difference between the LO frequency and carrier frequency. The IF contains information on the amplitude, polarization and phase encoding of the input optical signal. Optical coherent receivers may be substantially integrated, e.g., on a semiconductor chip, for various applications, particularly in the telecommunications and test and measurement industries. An integrated optical coherent receiver reduces the cost and complexity of signal reception and analysis of amplitude, phase and polarization modulated optical signals used in modern photonic transport networks.
Conventional integrated optical coherent receivers include the LO laser and corresponding laser cavity on the same chip as the other receiver components, such as semiconductor optical amplifiers (SOAs), waveguides, optical hybrid and photodetectors. An on-chip LO laser is typically a tunable sampled-grating distributed Bragg reflector (SG-DBR) laser. Although an on-chip laser cavity reduces overall size of the integrated optical coherent receiver, the phase noise reduces the receiver performance.
The LO laser is a key component of an optical coherent receiver. The phase and intensity noise properties of the LO laser generally govern performance of the optical coherent receiver. The phase noise in particular impacts minimum bit rates and the complexity of the phase modulation that may be applied to the input optical signal, while maintaining good reception. When the phase of the LO laser varies due to phase noise in the LO laser cavity, it imparts the variation on the receiver signals, including the input optical signal and detected signals, which leads to error in estimated signaling phase, for example. Efforts to improve the phase noise of LO lasers in integrated receivers include running at higher power levels, to the extent that rebroadening of the laser linewidth does not occur. Also, injection locking is used, where the LO laser is locked to a reference laser source with lower phase noise, where the reference laser source is off-chip.